The present invention relates to a method for assembling and testing electronic parts using Pb-free (lead-free) solder alloy, and an electronic circuit baseboard manufactured and tested by the method.
In particular, the present invention relates to an electronic circuit baseboard assembling and testing method capable of precisely testing conduction of the parts using a test land while suppressing oxidation thereof.
In the past, an Sn—Pb (tin-lead) type solder, including a large quantity of Pb (lead), is generally utilized when electronic parts are mounted. However, when an electronic circuit baseboard is soldered with an Sn—Pb type solder and the lead is discarded, the solder sometimes fuses out therefrom, giving undesirable effects to an ecological system and thereby causing environmental pollution. As a result, usage of a Pb-free type solder alloy is highly desirable.
After investigations of various Pb-free solder alloys; three components such as an Sn—Ag—Bi (tin-silver-bismuth) based material is a prevailing candidate for a Pb-free type solder alloy rather than an Sn—Pb type solder.
The reason is that various compositions formed by a two-component type solder alloy have already been examined as Pb-free solder alternatives. For example, since Sn-3,5 weight % Ag has a fusing point of 221° C. and Sn-5 weight % Sb (antimony) has a fusing point of 199° C., respectively, these fusing points are too high in comparison to the Sn-37 weight % Pb solder alloy. The Sn-37 weight % Pb has a fusing point of 183° C.
Accordingly, these two component type materials are not employed as Pb-free solders for a conventional glass epoxy baseboard. In addition, even though Sn-9 weight % Zn (zinc) has a low fusing point of 199° C., the solder's surface is easily oxidized. The solder's surfaces wetting performance, with regard to an electrode comprising Cu (copper) or Ni (nickel) is particularly low in comparison to that of an Sn—Ag or Sn—Sb type solder. As a result, Sn-9 weight % Zn is not employed either as a Pb-free type solder. Furthermore, since Sn-58 weight % Bi has a fusing point of 138 C and is hard and brittle, this two component type alloy has problems associated with its structural integrity and is thus difficult to employ. Sn-52 weight % In (indium) also has a low fusing point of 117° C. relative to Sn-37 weight % Pb which has a fusing point of 183° C., This difference in fusing point temperatures causes an additional problem of a weakening intensity in the solder connection section at high temperatures. In contrast, the fusing point can be approximated more closely, to 183° C. (e.g. the fusing point of Sn-37 weight % Pb) when a three component type Pb-free alloy, such as Sn—Ag—Bi is employed, as compared to when a two component type Pb-free alloy is employed.
However, when seeking prescribed materials whose fusing points approximate 183° C., in the three component type Pb-free alloy, a perfect eutectic composition is not obtained. A composition should have a solid and liquid coexisting temperature (e.g. a solid phase line temperature lower than 183° C. and a liquid phase line temperature higher than 183° C.). Thus, when a flow soldering process is performed after parts are connected by a reflow soldering process, and air-cooling is performed without a blower for the baseboard, the respective temperatures decline at different rates in these added parts and the baseboard, As a result, a large temperature difference arises in the solder of the connecting sections since the connected parts have different heat capacities from that of the glass epoxy baseboard. In these situations, when a solder having a wide temperature range of a solid and liquid coexistence is utilized, the solder coagulates, because a phase having a low fusing point (e.g., a hard and brittle phase largely including Bi) is segregated at a higher temperature side. As a result, the connection strength of various parts which complete the segregation phase after receiving a reflow soldering process is readily weakened.
To resolve such a problem, an Sn—Ag—Cu three composition type PB free solder alloy, which is excellent at solder connection credibility, can be used. However, a melting point of the Su—Ag—Cu solder alloy is 217° C. and is still considerably higher than that of conventional Sn-37 mass % Pb having a melting point 183° C. Thus, when such a higher melting point solder is coated on a land, formed from a copper leaf of an electronic circuit baseboard, the copper leaf section is easily oxidized, and a wetting performance of such a solder is low. As a result, there exists difficulty in testing electrical conduction of an electronic parts by contacting a tester to the land after soldering.